In recent years, hybrid vehicles that run by efficiently combining an internal combustion engine (engine) and an electric motor (motor) have been put into practical use in view of environmental issues. For example, Japanese Patent Laying-Open No. 2007-76551 (Patent Document 1) discloses an engine including an exhaust gas recirculation (hereinafter referred to as EGR) device for recirculating part of exhaust gas in an exhaust pipe to an intake pipe again, as an engine incorporated in such a hybrid vehicle.
This EGR device recirculates part of exhaust gas exhausted from an engine, and mixes the recirculated gas with a new air-fuel mixture to lower a combustion temperature, thereby suppressing production of nitrogen oxides (NOx) and suppressing pumping loss to improve fuel efficiency.
For further improvement in fuel efficiency of the engine including the EGR device, it has also been studied to increase an amount of the exhaust gas recirculated to the intake pipe (EGR gas), namely, to increase an EGR ratio, which is a ratio of the EGR gas to an amount of intake air introduced into a cylinder. This is because increase in EGR ratio allows combustion with a stoichiometric air-fuel ratio even when fuel and oxygen have low densities, and can suppress increase in an amount of fuel injection for adjusting a temperature of the exhaust gas because combustion heat is absorbed by the exhaust gas in the air-fuel mixture.    Patent Document 1: Japanese Patent Laying-Open No. 2007-76551